A vehicle may be propelled by an electric drive motor, an engine, or both. In certain industrial, construction, earth-moving, transportation, mass-transit, mining, agricultural, and other applications of drive trains, the load on the engine or drive motor may fluctuate. For example, for many applications, the load on the engine of a hybrid vehicle may increase as the vehicle travels up a hill or up sloped terrain. Accordingly, there is a need to provide sufficient electric energy to the electric drive motor to compensate for such fluctuations in load.